Zoom lens and video camera comprising the same

ABSTRACT

A zoom lens includes: a first lens group including a negative lens, a positive lens, and a positive meniscus lens having a convex surface on an object side; a second lens group including a negative lens and a cemented lens composed of a double-concave lens and a positive lens; a third lens group including a positive lens and a negative plastic lens, and having at least one aspherical surface; a fourth lens group including a cemented lens composed of a negative plastic lens and a positive plastic lens, and having at least one aspherical surface. These lenses are arranged in the stated order from an object side. In this zoom lens, the following expression is satisfied: 
     
       
         5&lt;|( fp   1+   fp   2+   fp   3)/   fw |&lt;12 
       
     
     where fp 1  represents a focal length of the negative plastic lens of the third lens group, fp 2  and fp 3  represent a focal length of the negative plastic lens and a focal length of the positive plastic lens of the fourth lens group, respectively, and fw represents a combined focal length of the entire system at a wide position.

TECHNICAL FIELD

The present invention relates to a zoom lens and a video camera usingthe same. More specifically, the present invention relates to ahigh-magnification spherical zoom lens that achieves a highmagnification (zoom ratio: 23 times), high brightness (an F number of1.6), low cost and a long back-focus, as well as to a video camera usingthe same.

BACKGROUND ART

Conventionally, to reduce the production cost of zoom lenses, plasticmaterials are used often as a lens material. Besides, in recent years,in the development of zoom lenses, in order to be competitive in themarket, a zoom lens having a high resolution power while having a highvarying power strongly has been demanded. In other words, it isnecessary to provide a zoom lens with a high varying power and highresolution that can be produced at low cost.

A zoom lens that incorporates a plastic lens is disclosed in, forexample, JP 8(1996)-106046 A, JP 9(1997)-311272 A. JP 8(1996)-106046 Adiscloses a zoom lens including ten lenses, four of which are plasticlenses, to provide a zoom ratio of 12 times. Furthermore, JP9(1997)-311272 A discloses a zoom lens including ten lenses, five ofwhich are plastic lenses, to provide a zoom ratio of 18 times.

However, in a zoom lens having a zoom ratio of 20 times or more, if aplastic lens is employed, the zoom lens incurs a great change inrefractive indices of plastic materials due to a temperature change.Thus, since it is difficult to apply plastic lenses in ahigh-magnification zoom lens, most of lenses composing a zoom lens areglass lenses in the current state.

DISCLOSURE OF THE INVENTION

Therefore, with the foregoing in mind, it is an object of the presentinvention to provide a zoom lens that achieves high brightness at an Fnumber of 1.6, a high magnification at a zoom ratio of 23 times, as wellas high performance and low cost by applying an optimal powerarrangement and an optimal arrangement of plastic lenses, and also toprovide a video camera employing the foregoing zoom lens.

To achieve the foregoing object, a zoom lens according to a first aspectof the present invention includes: a first lens group having positiverefracting power and being fixed with respect to the image plane; asecond lens group having negative refracting power and varying power bymoving along an optical axis; a third lens group having positiverefracting power and being fixed with respect to the image plane; and afourth lens group having positive refracting power and moving along theoptical axis so that the image plane varied by a movement of the secondlens group and a movement of an object is kept at a predeterminedposition from a reference plane. The first, second, third, and fourthlens groups are arranged in this order from an object side to an imageplane side. In the zoom lens, the first lens group includes a negativelens, a positive lens, and a positive meniscus lens arranged from theobject side in this order, in which the positive meniscus lens has aconvex surface on the object side. The second lens group includes anegative lens, a double-concave lens, and a positive lens arranged fromthe object side in this order, and includes at least one asphericalsurface, in which the double-concave lens and the positive lens arecemented with each other. The third lens group includes a positive lensand a negative plastic lens arranged from the object side in this order,and includes at least one aspherical surface. The fourth lens groupincludes a negative plastic lens and a positive plastic lens that arearranged from the object side in this order and cemented with eachother, and includes at least one aspherical surface. In this zoom lens,the following expression (36) is satisfied:

5<|(fp 1+fp 2 +fp 3)/fw|<12  (36)

where fp1 represents a focal length of the negative plastic lens of thethird lens group, fp2 represents a focal length of the negative plasticlens of the fourth lens group, fp3 represents a focal length of thepositive plastic lens of the fourth lens group, and fw represents acombined focal length of the entire system at a wide position.

With the above configuration of the zoom lens of the first aspect, it ispossible to provide a zoom lens with a high magnification at a zoomratio of 20 times or more, while balancing various aberrations thereofwell. Besides, it is possible to cancel changes in respective refractiveindices of plastic lens materials caused by temperature changes, therebyreducing deviations of the position of the image plane.

Furthermore, in the zoom lens according to the first aspect of thepresent invention, the following expression (37) preferably issatisfied:

7<|(fp 1 +fp 2 +fp 3)/fw|<10.5  (37)

Using this preferable example, it is possible to cancel changes in therespective refractive indices of the plastic lens materials caused bytemperature changes, thereby substantially eliminating deviations of theposition of the image plane. In this case, furthermore, the followingexpressions (38) to (41) preferably are satisfied:

9<f 1/fw<11  (38)

1<|f 2 /fw|<2  (39)

4.5<f 3 /fw<6  (40)

4.5<f 4/fw<6.5  (41)

where f1 represents a combined focal length of the first lens group, f2represents a combined focal length of the second lens group, f3represents a combined focal length of the third lens group, and f4represents a combined focal length of the fourth lens group.

Using this preferable example, it is possible to make the zobm lenscompact, while adjusting the various aberration performancesexcellently. In this. case, furthermore, the following expression (42)preferably is satisfied:

d 12 ×fw<1.2  (42)

where d12 represents a distance between the positive lens and thenegative plastic lens of the third lens group.

Using this preferable example, a chromatic aberration can be correctedexcellently in a zooming range from the wide position to a teleposition.

Furthermore, in the zoom lens according to the first aspect of thepresent invention, the following expression (43) preferably issatisfied:

(sag(r 1)+sag(r 2)+d 8)/d 8<4.5  (43)

where sag (r1) represents a sag amount between a center of an incidentsurface of the double-concave lens of the second lens group and aposition where the incident surface of the double-concave lens isbrought into contact with an outgoing surface of the negative lensdisposed on the object side in the second lens group, sag (r2)represents a sag amount between a center and an outer-most peripheralportion of the outgoing surface of the double-concave lens, and d8denotes a thickness of the double-concave lens.

Using this preferable example, the double-concave lens can be formedreadily, whereby the yield thereof can be improved.

Furthermore, in the zoom lens according to the first aspect of thepresent invention, it is preferable that a radius of curvature of a lenssurface closest to the image plane of the first lens group and a radiusof curvature of a lens surface closest to the object of the second lensgroup are equal to each other. Using this preferable example, it ispossible to prevent a distance between the surface closest to the imageplane of the first lens group and the surface closest to the object ofthe second lens group from decreasing with increasing proximity to alens periphery. This facilitates the production of a lens barrel.

Furthermore, in the zoom lens according to the first aspect of thepresent invention, the following expression (44) preferably issatisfied:

0.6<BF/fw<1.1  (44)

where BF represents an air distance between an image-plane-side surfaceof the lens closest to the image plane and the image plane.

Using this preferable example, it is possible to ensure a back-focusnecessary for allowing an infrared cut-off filter or a low-pass filtersuch as a crystal filter to be inserted. Besides, the back-focus isprevented from increasing unnecessarily, which makes it possible toprovide a compact zoom lens.

Furthermore, a zoom lens according to a second aspect of the presentinvention includes: a first lens group having positive refracting powerand being fixed with respect to the image plane; a second lens grouphaving negative refracting power and varying power by moving along anoptical axis; a third lens group having positive refracting power andbeing fixed with respect to the image plane; and a fourth lens grouphaving positive refracting power and moving along the optical axis sothat the image plane varied by a movement of the second lens group and amovement of an object is kept at a predetermined position from areference plane. The first, second, third, and fourth lens groups arearranged in this order from an object side to an image plane side. Inthe zoom lens, the first lens group includes a negative lens, a positivelens, and a positive meniscus lens arranged from the object side in thisorder, in which the positive meniscus lens has a convex surface on theobject side. The second lens group includes a negative lens, adouble-concave lens, and a positive lens arranged from the object sidein this order, and includes at least one aspherical surface, in whichthe double-concave lens and the positive lens are cemented with eachother. The third lens group includes a positive lens and a negativeplastic lens arranged from the object side in this order, and includesat least one aspherical surface. The fourth lens group includes apositive plastic lens and a negative plastic lens that are arranged fromthe object side in this order and cemented with each other, and includesat least one aspherical surface. In this zoom lens, the followingexpression (45) is satisfied:

5<|(fp 1 +fp 2 +fp 3)/fw|<12  (45)

where fp1 represents a focal length of the negative plastic lens of thethird lens group, fp2 represents a focal length of the positive plasticlens of the fourth lens group, fp3 represents a focal length of thenegative plastic lens of the fourth lens group, and fw represents acombined focal length of the entire system at a wide position.

With the above configuration of the zoom lens of the second aspect, itis possible to provide a zoom lens with a high magnification at a zoomratio of 20 times or more, while balancing various aberrations thereofwell. Besides, it is possible to cancel changes in respective refractiveindices of plastic lens materials caused by temperature changes, therebyreducing deviations of the position of the image plane.

Furthermore, in the zoom lens according to the second aspect of thepresent invention, the following expression (46) preferably issatisfied:

7<|(fp 1 +fp 2 +fp 3)/fw|<10.5  (46)

Using this preferable example, it is possible to cancel changes in therespective refractive indices of the plastic lens materials caused bytemperature changes, thereby substantially eliminating deviations of theposition of the image plane. In this case, furthermore, the followingexpressions (47) to (50) preferably are satisfied:

9<f 1 /fw<11  (47)

1<|f 2/fw1<2  (48)

4.5<f 3/fw<6  (49)

4.5<f 4/fw<6.5  (50)

where f1 represents a combined focal length of the first lens group, f2represents a combined focal length of the second lens group, f3represents a combined focal length of the third lens group, and f4represents a combined focal length of the fourth lens group.

Using this preferable example, it is possible to make the zoom lenscompact, while adjusting the aberrations excellently. In this case,furthermore, the following expression (51) preferably is satisfied:

d 12×fw<1.2  (51)

where d12 represents a distance between the positive lens and thenegative plastic lens of the third lens group.

Using this preferable example, a chromatic aberration can be correctedexcellently in a zooming range from the wide position to a teleposition.

Furthermore, in the zoom lens according to the second aspect of thepresent invention, the following expression (52) preferably issatisfied:

(sag(r 1)+sag(r 2)+d 8)/d 8<4.5  (52)

where sag (r1) represents a sag amount between a center of an incidentsurface of the double-concave lens of the second lens group and aposition where the incident surface of the double-concave lens isbrought into contact with an outgoing surface of the negative lensdisposed on the object side in the second lens group, sag (r2)represents a sag amount between a center and an outer-most peripheralportion of the outgoing surface of the double-concave lens, and d8denotes a thickness of the double-concave lens.

Using this preferable example, the double-concave lens can be formedreadily, whereby the yield thereof can be improved.

Furthermore, in the zoom lens according to the second aspect of thepresent invention, it is preferable that a radius of curvature of a lenssurface closest to the image plane of the first lens group and a radiusof curvature of a lens surface closest to the object of the second lensgroup are equal to each other Using this preferable example, it ispossible to prevent a distance between the surface closest to the imageplane of the first lens group and the surface closest to the object ofthe second lens group from decreasing with increasing proximity to alens periphery. This facilitates the production of a lens barrel.

Furthermore, in the zoom lens according to the second aspect of thepresent invention, the following expression (53) preferably issatisfied:

0.6<BF/fw<1.1  (53)

where BF represents an air distance between an image-plane-side surfaceof the lens closest to the image plane and the image plane.

Using this preferable example, it is possible to ensure a back-focusnecessary for allowing an infrared cut-off filter or a low-pass filtersuch as a crystal filter to be inserted. Besides, the back-focus isprevented from increasing unnecessarily, which makes it possible toprovide a compact zoom lens.

Furthermore, a zoom lens according to a third aspect of the presentinvention includes: a first lens group having positive refracting powerand being fixed with respect to the image plane; a second lens grouphaving negative refracting power and varying power by moving along anoptical axis; a third lens group having positive refracting power andbeing fixed with respect to the image plane; and a fourth lens grouphaving positive refracting power and moving along the optical axis sothat the image plane varied by a movement of the second lens group and amovement of an object is kept at a predetermined position from areference plane. The first, second, third, and fourth lens groups arearranged in this order from an object side to an image plane side. Inthe zoom lens, the first lens group includes a negative lens, a positivelens, and a positive meniscus lens arranged from the object side in thisorder, in which the positive meniscus lens has a convex surface on theobject side. The second lens group includes a negative lens, adouble-concave lens, and a positive lens arranged from the object sidein this order, and includes at least one aspherical surface, in whichthe double-concave lens and the positive lens are cemented with eachother. The third lens group includes a positive lens and a negativeplastic lens arranged from the object side in this order, and includesat least one aspherical surface. The fourth lens group includes anegative plastic lens and a positive plastic lens that are arranged fromthe object side in this order, and includes at least one asphericalsurface. In this zoom lens, the following expression (54) is satisfied:

5<|(fp 1 +fp 2 +fp 3)/fw|<12  (54)

where fp1 represents a focal length of the negative plastic lens of thethird lens group, fp2 represents a focal length of the negative plasticlens of the fourth lens group, fp3 represents a focal length of thepositive plastic lens of the fourth lens group, and fw represents acombined focal length of the entire system at a wide position.

With the above configuration of the zoom lens of the third aspect, it ispossible to provide a zoom lens with a high magnification at a zoomratio of 20 times or more, while balancing various aberrations thereofwell. Besides, it is possible to cancel changes in respective refractiveindices of plastic lens materials caused by temperature changes, therebyreducing deviations of the position of the image plane.

Furthermore, in the zoom lens according to the third aspect of thepresent invention, the following expression (55) preferably issatisfied:

7<|(fp 1 +fp 2 +fp 3)/fw|<10.5  (55)

Using this preferable example, it is possible to cancel changes in therespective refractive indices of the plastic lens materials caused bytemperature changes, thereby substantially eliminating deviations of theposition of the image plane. In this case, furthermore, the followingexpressions (56) to (59) preferably are satisfied:

9<f 1 /fw<11  (56)

1<|f 2 /fw|<2  (57)

4.5<f 3/fw<6  (58)

4.5<f 4/fw<6.5  (59)

where f1 represents a combined focal length of the first lens group, f2represents a combined focal length of the second lens group, f3represents a combined focal length of the third lens group, and f4represents a combined focal length of the fourth lens group.

Using this preferable example, it is possible to make the zoom lenscompact, while adjusting the aberrations excellently. In this case,furthermore, the following expression (60) preferably is satisfied:

d 12 ×fw<12  (60)

where d12 represents a distance between the positive lens and thenegative plastic lens of the third lens group.

Using this preferable example, a chromatic aberration can be correctedexcellently in a zooming range from the wide position to a teleposition.

Furthermore, in the zoom lens according to the third aspect of thepresent invention, the following expression (61) preferably issatisfied:

(sag(r 1)+sag(r 2)+d 8)/d 8<4.5  (61)

where sag (r1) represents a sag amount between a center of an incidentsurface of the double-concave lens of the second lens group and aposition where the incident surface of the double-concave lens isbrought into contact with an outgoing surface of the negative lensdisposed on the object side in the second lens group, sag (r2)represents a sag amount between a center and an outer-most peripheralportion of the outgoing surface of the double-concave lens, and d8denotes a thickness of the double-concave lens.

Using this preferable example, the double-concave lens can be formedreadily, whereby the yield thereof can be improved.

Furthermore, in the zoom lens according to the third aspect of thepresent invention, it is preferable that a radius of curvature of a lenssurface closest to the image plane of the first lens group and a radiusof curvature of a lens surface closest to the object of the second lensgroup are equal to each other. Using this preferable example, it ispossible to prevent a distance between the surface closest to the imageplane of the first lens group and the surface closest to the object ofthe second lens group from decreasing with increasing proximity to alens periphery. This facilitates the production of a lens barrel.

Furthermore, in the zoom lens according to the third aspect of thepresent invention, the following expression (62) preferably issatisfied:

0.6<BF/fw<1.1  (62)

where BF represents an air distance between an image-plane-side surfaceof the lens closest to the image plane and the image plane.

Using this preferable example, it is possible to ensure a back-focusnecessary for allowing an infrared cut-off filter or a low-pass filtersuch as a crystal filter to be inserted. Besides, the back-focus isprevented from increasing unnecessarily, which makes it possible toprovide a compact zoom lens.

Furthermore, a zoom lens according to a fourth aspect of the presentinvention includes: a first lens group having positive refracting powerand being fixed with respect to the image plane; a second lens grouphaving negative refracting power and varying power by moving along anoptical axis; a third lens group having positive refracting power andbeing fixed with respect to the image plane; and a fourth lens grouphaving positive refracting power and moving along the optical axis sothat the image plane varied by a movement of the second lens group and amovement of an object is kept at a predetermined position from areference plane. The first, second, third, and fourth lens groups arearranged in this order from an object side to an image plane side. Inthe zoom lens, the first lens group includes a negative lens, a positivelens, and a positive meniscus lens arranged from the object side in thisorder, in which the positive meniscus lens has a convex surface on theobject side. The second lens group includes a negative lens, adouble-concave lens, and a positive lens arranged from the object sidein this order, and includes at least one aspherical surface, in whichthe double-concave lens and the positive lens are cemented with eachother. The third lens group includes a positive lens and a negativeplastic lens that are arranged from the object side in this order andcemented with each other, and includes at least one aspherical surface.The fourth lens group includes a negative plastic lens and a positiveplastic lens that are arranged from the object side in this order andcemented with each other, and includes at least one aspherical surface.In this zoom lens, the following expression (63) is satisfied:

5<|(fp 1 +fp 2 +fp 3)/fw|<12  (63)

where fp1 represents a focal length of the negative plastic lens of thethird lens group, fp2 represents a focal length of the negative plasticlens of the fourth lens group, fp3 represents a focal length of thepositive plastic lens of the fourth lens group, and fw represents acombined focal length of the entire system at a wide position.

With the above configuration of the zoom lens of the fourth aspect, itis possible to provide a zoom lens with a high magnification at a zoomratio of 20 times or more, while balancing various aberrations thereofwell. Besides, it is possible to cancel changes in respective refractiveindices of plastic lens materials caused by temperature changes, therebyreducing deviations of the position of the image plane.

Furthermore, in the zoom lens according to the fourth aspect of thepresent invention, the following expression (64) preferably issatisfied:

7<|(fp 1 +fp 2 +fp 3)/fw|<10.5  (64)

Using this preferable example, it is possible to cancel changes in therespective refractive indices of the plastic lens materials caused bytemperature changes, thereby substantially eliminating deviations of theposition of the image plane. In this case, furthermore, the followingexpressions (65) to (68) preferably are satisfied:

9<f 1/fw<11  (65)

1<|f 2/fw|<2  (66)

4.5<f 3/fw<6  (67)

4.5<f 4/fw<6.5  (68)

where f1 represents a combined focal length of the first lens group, f2represents a combined focal length of the second lens group, f3represents a combined focal length of the third lens group, and f4represents a combined focal length of the fourth lens group.

Using this preferable example, it is possible to make the zoom lenscompact, while adjusting the aberrations excellently.

Furthermore, in the zoom lens according to the fourth aspect of thepresent invention, the following expression (69) preferably issatisfied:

(sag(r 1)+sag(r 2)+d 8)/d 8<4.5  (69)

where sag (r1) represents a sag amount between a center of an incidentsurface of the double-concave lens of the second lens group and aposition where the incident surface of the double-concave lens isbrought into contact with an outgoing surface of the negative lensdisposed on the object side in the second lens group, sag (r2)represents a sag amount between a center and an outer-most peripheralportion of the outgoing surface of the double-concave lens, and d8denotes a thickness of the double-concave lens.

Using this preferable example, the double-concave lens can be formedreadily, whereby the yield thereof can be improved.

Furthermore, in the zoom lens according to the fourth aspect of thepresent invention, it is preferable that a radius of curvature of a lenssurface closest to the image plane of the first lens group and a radiusof curvature of a lens surface closest to the object of the second lensgroup are equal to each other.

Using this preferable example, it is possible to prevent a distancebetween the surface closest to the image plane of the first lens groupand the surface closest to the object of the second lens group fromdecreasing with increasing proximity to a lens periphery. Thisfacilitates the production of a lens barrel.

Furthermore, in the zoom lens according to the fourth aspect of thepresent invention, the following expression (70) preferably issatisfied:

0.6<BF/fw<1.1  (70)

where BF represents an air distance between an image-plane-side surfaceof the lens closest to the image plane and the image plane.

Using this preferable example, it is possible to ensure a back-focusnecessary for allowing an infrared cut-off filter or a low-pass filtersuch as a crystal filter to be inserted. Besides, the back-focus isprevented from increasing unnecessarily, which makes it possible toprovide a compact zoom lens.

Furthermore, a video camera according to the present invention isconfigured so as to include the zoom lens according to the presentinvention. With this configuration for the video camera, it is possibleto provide a video camera that is small in size, light in weight, andproduced at low cost.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a view showing a configuration of a zoom lens according toEmbodiment 1 of the present invention.

FIGS. 2A to 2E are views showing various aberrations at a wide positionof the zoom lens according to Embodiment 1 of the present invention.

FIGS. 3A to 3E are views showing various aberrations at a normalposition of the zoom lens according to Embodiment 1 of the presentinvention.

FIGS. 4A to 4E are views showing various aberrations at a tele positionof the zoom lens according to Embodiment 1 of the present invention.

FIG. 5 is a view showing a configuration of a zoom lens according toEmbodiment 2 of the present invention.

FIGS. 6A to 6E are views showing various aberrations at a wide positionof the zoom lens according to Embodiment 2 of the present invention.

FIGS. 7A to 7E are views showing various aberrations at a normalposition of the zoom lens according to Embodiment 2 of the presentinvention.

FIGS. 8A to 8E are views showing various aberrations at a tele positionof the zoom lens according to Embodiment 2 of the present invention.

FIG. 9 is a view showing a configuration of a zoom lens according toEmbodiment 3 of the present invention.

FIGS. 10A to 10E are views showing various aberrations at a wideposition of the zoom lens according to Embodiment 3 of the presentinvention.

FIGS. 11A to 11E are views showing various aberrations at a normalposition of the zoom lens according to Embodiment 3 of the presentinvention.

FIGS. 12A to 12E are views showing various aberrations at a teleposition of the zoom lens according to Embodiment 3 of the presentinvention.

FIG. 13 is a view showing a configuration of a zoom lens according toEmbodiment 4 of the present invention.

FIGS. 14A to 14E are views showing various aberrations at a wideposition of the zoom lens according to Embodiment 4 of the presentinvention.

FIGS. 15A to 15E are views showing various aberrations at a normalposition of the zoom lens according to Embodiment 4 of the presentinvention.

FIGS. 16A to 16E are views showing various aberrations at a teleposition of the zoom lens according to Embodiment 4 of the presentinvention.

FIG. 17 is a view showing a configuration of a video camera according toEmbodiment 5 of the present invention.

DESCRIPTION OF THE INVENTION

Hereinafter, the present invention will be described by way ofillustrative embodiments with reference to the drawings.

[Embodiment 1]

FIG. 1 is a view showing the arrangement of a zoom lens according toEmbodiment 1 of the present invention.

As shown in FIG. 1, the zoom lens has a structure in which a first lensgroup 11, a second lens group 12, a third lens group 13, a fourth lensgroup 14, and a glass plate 15 are arranged from an. object side (leftside in FIG. 1) to an image plane 16 side (right side in FIG. 1) in thisorder. Here, the glass plate 15 is equivalent optically to a crystalfilter, a face plate of an imaging device, etc.

The first lens group 11 has positive refracting power, and is fixed withrespect to the image plane 16 even when varying power and focusing. Thesecond lens group 12 has negative refracting power and varies power bymoving along an optical axis The third lens group 13 has positiverefracting power, and is fixed with respect to the image plane 16 whenvarying power and focusing. The fourth lens group 14 has positiverefracting power, and moves along the optical axis so that the imageplane 16 varied by the movement of the second lens group 12 and themovement of the object to be imaged is kept at a predetermined positionfrom a reference plane, thereby moving an image and adjusting the focusthereof at the same time in accordance with variable power.

The first lens group 11 is composed of a negative lens 1 a, a positivelens 1 b, and a positive meniscus lens 1 c arranged from the object sidein this order, in which the positive meniscus lens 1 c has a convexsurface on the object side. The second lens group 12 is composed of anegative lens 2 a, and a cemented lens of a double-concave lens 2 b anda positive lens 2 c, which are arranged from the object side in thisorder, in which at least one of the surfaces of the foregoing lenses isaspherical. The third lens group 13 is composed of a positive lens 3 aand a negative plastic lens 3 b arranged from the object side in thisorder, in which at least one of the surfaces of these lenses isaspherical. The fourth lens group 14 is a cemented lens composed of anegative plastic lens 4 a and a positive plastic lens 4 b that arearranged from the object side in this order, in which at least one ofthe surfaces of these lenses is aspherical.

In the zoom lens according to the present embodiment, the followingexpression (71) is satisfied:

5<|(fp 1 +fp 2 +fp 3)/fw|<12  (71)

where fp1 represents a focal length of the negative plastic lens 3 b ofthe third lens group 13, fp2 represents a focal length of the negativeplastic lens 4 a of the fourth lens group 14, fp3 represents a focallength of the positive plastic lens 4 b of the fourth lens group 14, andfw represents a combined focal length of the entire system at a wideposition.

With such a configuration that satisfies the expression (71), changes inrefractive indices of the plastic lens materials caused by temperaturechanges can be canceled, whereby a deviation of the image plane positioncan be decreased. Generally, as properties of a plastic material, arefractive index thereof decreases as the temperature rises andincreases as the temperature falls, and the plastic material swells asthe temperature rises and shrinks as the temperature falls. In otherwords, if |(fp1+fp2+fp3)/fw| is not more than the lower limit of theexpression (71), a negative-lens tendency increases in the combinedfocal length of the focal length fp1 of the negative plastic lens 3 b ofthe third lens group 13, and the focal length fp2 of the negativeplastic lens 4 a and the focal length fp3 of the positive plastic lens 4b of the fourth lens group 14, and with a temperature rise, the imageplane position is deviated farthest on the object side at the wideposition. On the contrary, with a temperature fall, the image planeposition is deviated significantly toward the image plane side at thewide position. This causes a phenomenon in which the fourth lens group14 moving along the optical axis within a certain moving range so as tokeep the image plane at a predetermined position from a referencesurface is incapable of doing so as long as it moves within theforegoing moving range, thereby resulting in defocusing. On the otherhand, if |(fp1+fp2+fp3)/fw| is not less than the upper limit of theexpression (71), a positive-lens tendency increases in the combinedfocus length of the focus length fp1 of the-negative-plastic lens 3 b ofthe third lens group 13 and the focus length fp2 of the negative plasticlens 4 a and the focus length fp3 of the positive plastic lens 4 b ofthe fourth lens group 14, and with a temperature rise, the image planeposition is deviated farthest on the image plane side at the normalposition. Therefore, this also results in defocusing.

Furthermore, in the zoom lens of the present embodiment, the followingexpression (72) is satisfied.

7<|(fp 1 +fp 2 +fp 3)/fw|<10.5  (72)

With such a configuration of the zoom lens that satisfies the expression(72) above, changes in refractive indices of the plastic lens materialscaused by temperature changes are canceled, resulting in substantiallyno deviation of the image plane position occurring.

Furthermore, in the zoom lens according to the present embodiment, thefollowing expressions (73) to (76) desirably are satisfied:

9<f 1 /fw<11  (73)

1<|f 2 /fw|<2  (74)

4.5<f 3 /fw<6  (75)

4.5<f 4/fw<6.5  (76)

where f1 represents a combined focal length of the first lens group 11,f2 represents a combined focal length of the second lens group 12, f3represents a combined focal length of the third lens group 13, and f4represents a combined focal length of the fourth lens group 14.

In the case where the expressions (73) to (76) are satisfied, the zoomlens is configured to be compact, with aberration performancesexcellently adjusted.

If f1/fw is not more than the lower limit of the expression (73), thefirst lens group 11 has an excessive refracting power, which makes itdifficult to correct a spherical aberration at the side of the longfocal length and an off-axis coma-aberration. Besides, if f1/fw is notless than the upper limit of the expression (73), the length of theentire lens increases, which makes it difficult to make the zoom lenscompact.

If |f2/fw| is not more than the lower limit of the expression (74), aPetzval sum of the entire system increases, so that a field curvaturecannot be corrected. If |f2/fw| is not less than the upper limit of theexpression (74), the Petzval sum decreases, but the length of the entiresystem increases, which makes it difficult to make the zoom lenscompact.

If f3/fw is not more than the lower limit of the expression (75), therefracting power of the third lens group 13 increases, which makes itimpossible to secure a back-focus that allows a crystal filter or thelike to be inserted therein, and makes it difficult to correct thespherical aberration. Furthermore, if f3/fw is not less than the. upperlimit of the expression (75), a Petzval sum increases, thereby making itdifficult to correct a field curvature.

If f4/fw is not more than the lower limit of the expression (76), thesize of the entire lens system increases, which makes it difficult tomake the zoom lens compact. Furthermore, if f4/fw is not less than theupper limit of the expression (76), it is difficult to correct off-axisaberrations both in near photographing and in long-distancephotographing at the same time.

Furthermore, in the zoom lens according to the present embodiment, thefollowing expression (77) desirably is satisfied:

d 12 ×fw<1.2.  (77)

where d12 represents a distance between the positive lens 3 a and thenegative plastic lens 3 b of the third lens group 13.

In the case where the expression (77) is satisfied, a chromaticaberration can be corrected excellently in a zooming range from the wideposition to the tele position. If d12×fw is not less than the upperlimit of the expression (77), the chromatic aberration varies moresignificantly from the wide position to the tele position, therebysignificantly deteriorating the performance.

In the zoom lens according to the present embodiment, the followingexpression (78) desirably is satisfied:

 (sag(r 1)+sag(r 2)+d 8)/d 8<4.5  (78)

where

sag (r1) represents a sag amount between the center of an incidentsurface of the double-concave lens 2 b of the second lens group 12 and aposition where the incident surface of the double-concave lens 2 b isbrought into contact with an outgoing surface of the negative lens 2 adisposed on the object side in the second lens group 12,

sag (r2) represents a sag amount between the center and an outer-mostperipheral portion of the outgoing surface of the double-concave lens 2b, and

d8 denotes a thickness of the double-concave lens 2 b.

With satisfaction of the expression (78), the double-concave lens 2 bcan be formed readily, whereby the yield thereof can be improved. If(sag(r1)+sag(r2)+d8)/d8 is not less than the upper limit of theexpression (78), the ratio of a thickness of the central portion of thelens to an edge thickness of the peripheral portion of the lensincreases, making it difficult to mold a lens. As a result, the yield islowered and a low cost of lenses cannot be realized.

Furthermore, desirably the zoom lens according to the present embodimentis configured so that a lens surface closest to the image plane of thefirst lens group 11 has a radius of curvature equal to a radius ofcurvature of a lens surface closest to the object of the second lensgroup 12. This prevents the distance between the surface closest to theimage plane of the first lens group 11 and the surface closest to theobject of the second lens group 12 from decreasing with increasingproximity to a lens periphery. This facilitates the production of a lensbarrel.

Furthermore, in the zoom lens according to the present embodiment, thefollowing expression (79) desirably is satisfied:

0.6<BF/fw<1.1  (79)

where BF represents an air distance between an image-plane-side surfaceor the lens closest to the image plane and the image plane.

By satisfying the foregoing expression (79), it is possible to ensure aback-focus necessary for allowing an infrared cut-off filter or alow-pass filter such as a crystal filter to be inserted. Besides, theback-focus is prevented from increasing unnecessarily, which makes itpossible to provide a compact zoom lens If BF/fw is not more than thelower limit of the expression (79), a distance sufficient for allowingan infrared cut-off filter or a low-pass filter such as a crystal filterto be inserted cannot be ensured. On the other hand, if BF/fw is notless than the upper limit of the expression (79), the back-focusexcessively increases, thereby making it impossible to provide a compactzoom lens.

EXAMPLE 1

The following Table 1 shows a specific example of the zoom lensaccording to the present embodiment.

TABLE 1 Group Surface rd th nd ν 1  1 37.31 0.80 1.80518 25.4  2 20.085.05 1.58913 61.2  3 −277.05 0.15  4 18.82 2.75 1.60311 60.7  5 51.75variable 2  6 51.75 0.60 1.80500 39.6  7 4.37 2.71  8* −8.59 0.801.60602 57.8  9 5.51 2.20 1.80518 25.5 10 71.99 variable 3 11* 8.42 3.701.60602 57.8 12* −10.17 0.20 13 −15.57 0.60 1.58387 30.1 14 15.57variable 4 15* 9.60 0.60 1.58387 30.1 16 4.64 2.70 1.49178 57.2 17*−18.52 variable 5 18 ∞ 2.80 1.51633 64.1 19 ∞ —

In Table 1, rd represents a radius of curvature (mm) of a lens, threpresents a thickness (mm) of a lens or an air distance (mm) betweenlenses, nd represents a refractive index of each lens with respect to ad-line, and ν represents an abbe number of each lens with respect to thed-line. The shape of an aspherical surface (in Table 1, such a surfaceis denoted with a mark * attached beside its reference number) isdefined by the following equation (80). $\begin{matrix}{Z = {\frac{{cy}^{2}}{1 + \sqrt{1 - {\left( {1 + k} \right)c^{2}y^{2}}}} + {Dy}^{4} + {Ey}^{6} + {Fy}^{8} + {Gy}^{10}}} & (80)\end{matrix}$

where y represents a height from the optical axis, Z represents adistance between a point on the aspherical surface at the height y fromthe optical axis and a tangent plane of the apex on the asphericalsurface, c represents a curvature at the apex on the aspherical surface,k represents a conical constant, and D, E, F, and G represent asphericalcoefficients.

The following Table 2 shows aspherical coefficients of the zoom lens inthe present example.

TABLE 2 Surface k D E F G 8 −11.79950 −2.20951 × 10⁻³  1.33194 × 10⁻⁴−1.25908 × 10⁻⁵  5.36379 × 10⁻⁷ 11 0.66449 −3.12933 × 10⁻⁴ −2.19407 ×10⁻⁶  2.99348 × 10⁻⁷ −5.45227 × 10⁻⁹ 12 0.68418  4.94313 × 10⁻⁴  2.82004× 10⁻⁶  4.37043 × 10⁻⁷ −8.94886 × 10⁻⁹ 15 −0.87201  4.78208 × 10⁻⁵−8.02361 × 10⁻⁶  2.23438 × 10⁻⁶ −1.34988 × 10⁻⁷ 17 −66.19940 −1.16522 ×10⁻³  6.85576 × 10⁻⁵ −9.23566 × 10⁻⁷ −1.35439 × 10⁻⁷

The following Table 3 shows an air distance (mm) that is varied byzooming in the case where an object is positioned at infinity.

TABLE 3 Wide position Normal position Tele position Focal length 3.01027.036 69.075 F No. 1.688 2.551 3.378 Angle of 65.136 7.614 2.954 view(2ω) th5 0.700 16.949 20.341 th10 21.740 5.491 2.099 th12 8.120 2.4908.120 th17 2.000 7.630 2.000

The normal position in Table 3 is where the third lens group 13 isplaced most closely to the fourth lens group 14 In Table 3, Focal length(mm), F No., and ω(°) represent a focal length, an F number, and anincident angle of view at a wide position, a normal position, and a teleposition of the zoom lens of the present example.

FIGS. 2A to 2E, 3A to 3E, and 4A to 4E show performances regardingvarious aberrations at the wide position, the normal position, and thetele position of the zoom lens shown in the present example,respectively. FIGS. 2A, 3A and 4A show a spherical aberration (mm);FIGS. 2B, 3B and 4B show astigmatism (mm); FIGS. 2C, 3C and 4C show adistortion aberration (%); FIGS. 2D, 3D and 4D show a longitudinalchromatic aberration (mm); and FIGS. 2E, 3E and 4E show a chromaticaberration of magnification (mm). In FIGS. 2B, 3B and 4B showingastigmatism, a solid line represents a sagittal feld curvature, and abroken line represents a meridional field curvature. In FIGS. 2D, 3D and4D showing the longitudinal chromatic aberration and FIGS. 2E, 3E and 4Eshowing the chromatic aberration of magnification, a solid linerepresents the values with respect to the d-line, a short broken linerepresents the values with respect to an F-line, and a long broken linerepresents the values with respect to a C-line. As is apparent from thedrawings regarding these aberrations, the zoom lens of the presentexample has an excellent aberration performance.

The amount of movement of the image plane position according to a changein a refractive index of a plastic lens material caused by a temperaturechange is 0.9 μm/C.° when the object is positioned at infinity and thezooming position is at the wide position.

[Embodiment 2]

FIG. 5 is a view showing the arrangement of a zoom lens according toEmbodiment 2 of the present invention.

As shown in FIG. 5, the zoom lens has a structure in which a first lensgroup 21, a second lens group 22, a third lens group 23, a fourth lensgroup 24, and a glass plate 25 are arranged from an object side (leftside in FIG. 5) to an image plane 26 side (right side in FIG. 5) in thisorder. Here, the glass plate 25 is equivalent optically to a crystalfilter, a face plate of an imaging device, etc.

The first lens group 21 has positive refracting power, and is fixed withrespect to the image plane 26 even when varying power and focusing. Thesecond lens group 22 has negative refracting power and varies power bymoving along an optical axis. The third lens group 23 has positiverefracting power, and is fixed with respect to the image plane 26 evenwhen varying power and focusing. The fourth lens group 24 has positiverefracting power, and moves along the optical axis so that the imageplane 26 varied by the movement of the second lens group 22 and themovement of the object to be imaged is kept at a predetermined positionfrom a reference plane, thereby moving an image and adjusting the focusthereof at the same time in accordance with variable power.

The first lens group 21 is composed of a negative lens 5 a, a positivelens 5 b, and a positive meniscus lens 5 c arranged from the objectside. in this order, in which the positive meniscus lens 5 c has aconvex surface on the object side. The second lens group 22 is composedof a negative lens 6 a, and a cemented lens of a double-concave lens 6 band a positive lens 6 c, which are arranged from the object side in thisorder, in which at least one of the surfaces of the foregoing lenses isaspherical. The third lens group 23 is composed of a positive lens 7 aand a negative plastic lens 7 b arranged from the object side in thisorder, in which at least one of the surfaces of these lenses isaspherical. The fourth lens group 24 is a cemented lens composed of apositive plastic lens 8 a and a negative plastic lens 8 b that arearranged from the object side in this order, in which at least one ofthe surfaces of these lenses is aspherical.

In the zoom lens according to the present embodiment, the followingexpression (81) is satisfied:

5<|(fp 1 +fp 2 +fp 3)/fw|<12  (81)

where fp1 represents a focal length of the negative plastic lens 7 b ofthe third lens group 23, fp2 represents a focal length of the positiveplastic lens 8 a of the fourth lens group 24, fp3 represents a focallength of the negative plastic lens 8 b of the fourth lens group 24, andfw represents a combined focal length of the entire system at a wideposition.

With such a configuration that satisfies the expression (81), changes inrefractive indices of the plastic lens materials caused by temperaturechanges can be canceled, whereby a deviation of the image plane positioncan be decreased. Generally, as properties of a plastic material, arefractive index thereof decreases as the temperature rises andincreases as the temperature falls, and the plastic material swells asthe temperature rises and shrinks as the temperature falls. In otherwords, if |(fp1+fp2+fp3)/fw| is not more than the lower limit of theexpression (81), a negative-lens tendency increases in the combinedfocal length of the focal length fp1 of the negative plastic lens 7 b ofthe third lens group 23, and the focal length fp2 of the positiveplastic lens 8 a and the focal length fp3 of the negative plastic lens 8b of the fourth lens group 24, and with a temperature rise, the imageplane position is deviated farthest on the object side at the wideposition. On the contrary, with a temperature fall, the image planeposition is deviated significantly toward the image plane side at thewide position. This causes a phenomenon in which the fourth lens group24 moving along the optical axis within a certain moving range so as tokeep the image plane at a predetermined position from a referencesurface is incapable of doing so as long as it moves within theforegoing moving range, thereby resulting in defocusing. On the otherhand, if |(fp1+fp2+fp3)/fw| is not less than the upper limit of theexpression (81), a positive-lens tendency increases in the combinedfocus length of the focus length fp1 of the negative plastic lens 7 b ofthe third lens group 23 and the focus length fp2 of the positive plasticlens 8 a and the focus length fp3 of the negative plastic lens 8 b ofthe fourth lens group 24, and with a temperature rise, the image planeposition is deviated farthest on the image plane side at the normalposition. Therefore, this also results in defocusing.

Furthermore, in the zoom lens of the present embodiment, the followingexpression (82) is satisfied.

7<|(fp 1 +fp 2 +fp 3)/fw|<10.5  (82)

With such a configuration of the zoom lens that satisfies the expression(82) above, changes in refractive indices of the plastic lens materialscaused by temperature changes are canceled, resulting in substantiallyno deviation of the image plane position occurring.

Furthermore, in the zoom lens according to the present embodiment, thefollowing expressions (83) to (86) desirably are satisfied:

9<f 1 /fw<11  (83)

1<|f 2/fw|<2  (84)

4.5<f 3/fw<6  (85)

4.5<f 4/fw<6.5  (86)

where f1 represents a combined focal length of the first lens group 21,f2 represents a combined focal length of the second lens group 22, f3represents a combined focal length of the third lens group 23, and f4represents a combined focal length of the fourth lens group 24.

In the case where the expressions (83) to (86) are satisfied, the zoomlens is configured to be compact, with aberration performancesexcellently adjusted.

If f1/fw is not more than the lower limit of the expression (83), thefirst lens group 21 has an excessive refracting power, which makes itdifficult to correct a spherical aberration at the side of the longfocal length and an off-axis coma-aberration. Besides, if f1/fw is notless than the upper limit of the expression (83), the full length of thelens increases, which makes it difficult to make the zoom lens compact.

If |f2/fw| is not more than the lower limit of the expression (84), aPetzval sum of the entire system increases, so that a field curvaturecannot be corrected. If |f2/fw| is not less than the upper limit of theexpression (84), the Petzval sum decreases, but the length of the entiresystem increases, which makes it difficult to make the zoom lenscompact.

If f3/fw is not more than the lower limit of the expression (85), therefracting power of the third lens group 23 increases, which makes itimpossible to secure a back-focus that allows a crystal filter or thelike to be inserted therein, and makes it difficult to correct thespherical aberration. Furthermore, if f3/fw is not less than the upperlimit of the expression (85), a Petzval sum increases, thereby making itdifficult to correct a field curvature.

If f4/fw is not more than the lower limit of the expression (86), thesize of the entire lens system increases, which makes it difficult tomake the zoom lens compact. Furthermore, if f4/fw is not less than theupper limit of the expression (86), it is difficult to correct off-axisaberrations both in near photographing and in long-distancephotographing at the same time.

Furthermore, in the zoom lens according to the present embodiment, thefollowing expression (87) desirably is satisfied:

d 12 ×fw<1.2  (87)

where d12 represents a distance between the positive lens 7 a and thenegative plastic lens 7 b of the third lens group 23.

In the case where the expression (87) is satisfied, a chromaticaberration can be corrected excellently in a zooming range from the wideposition to the tele position. If d12×fw is not less than the upperlimit of the expression (87), the chromatic aberration significantlyvaries from the wide position to the tele position, therebysignificantly deteriorating the performance.

In the zoom lens according to the present embodiment, the followingexpression (88) desirably is satisfied:

(sag(r 1)+sag(r 2)+d 8)/d 8<4.5  (88)

where

sag (r1) represents a sag amount between the center of an incidentsurface of the double-concave lens 6 b of the second lens group 22 and aposition where the incident surface of the double-concave lens 6 b isbrought into contact with an outgoing surface of the negative lens 6 adisposed on the object side in the second lens group 22,

sag (r2) represents a sag amount between the center and an outer-mostperipheral portion of the outgoing surface of the double-concave lens 6b, and,

d8 denotes a thickness of the double-concave lens 6 b.

With satisfaction of the expression (88), the double-concave lens 6 bcan be formed readily, whereby the yield thereof can be improved. If(sag(r1)+sag(r2)+d8)/d8 is not less than the upper limit of theexpression (88), the ratio of a thickness of the central portion of thelens to an edge thickness of the peripheral portion of the lensincreases, making it difficult to mold a lens. As a result, the yield islowered and a low cost of lenses cannot be realized.

Furthermore, desirably the zoom lens according to the present embodimentis configured so that a lens surface closest to the image plane of thefirst lens group 21 has a radius of curvature equal to a radius ofcurvature of a lens surface closest to the object of the second lensgroup 22. This prevents the distance between the surface closest to theimage plane of the first lens group 21 and the surface closest to theobject of the second lens group 22 from decreasing with increasingproximity to a lens periphery. This facilitates the production of a lensbarrel.

Furthermore, in the zoom lens according to the present embodiment, thefollowing expression (89) desirably is satisfied:

0.6<BF/fw<1.1  (89)

where BF represents an air distance between the image-plane-side surfaceof the lens closest to the image plane and the image plane.

By satisfying the foregoing expression (89), it is possible to ensure aback-focus necessary for allowing an infrared cut-off filter or alow-pass filter such as a crystal filter to be inserted. Besides, theback-focus is prevented from increasing unnecessarily, which makes itpossible to provide a compact zoom lens. If BF/fw is not more than thelower limit of the expression (89), a distance sufficient for allowingan infrared cut-off filter or a low-pass filter such as a crystal filterto be inserted cannot be ensured. On the other hand, if BF/fw is notless than the upper limit of the expression (89), the back-focusexcessively increases, thereby making it impossible to provide a compactzoom lens.

EXAMPLE 2

The following Table 4 shows a specific example of the zoom lensaccording to the present embodiment.

TABLE 4 Group Surface rd th nd ν 1  1 38.45 0.90 1.80518 25.4  2 20.525.10 1.58913 61.2  3 −183.44 0.15  4 18.54 2.70 1.60311 60.7  5 46.73variable 2  6 46.73 0.60 1.80500 39.6  7 4.34 2.75  8* −8.67 1.001.60602 57.8  9 5.50 2.30 1.80518 25.5 10 65.80 variable 3 11* 7.74 4.001.51450 63.5 12* −8.86 0.30 13 −16.83 0.60 1.58387 30.1 14 19.85variable 4 15* 16.72 2.80 1.54324 53.1 16 −4.90 0.70 1.58387 30.1 17*−13.59 variable 5 18 ∞ 2.80 1.51633 64.1 19 ∞ —

In Table 4, rd represents a radius of curvature (mm) of a lens, threpresents a thickness (mm) of a lens or an air distance (mm) betweenlenses, nd represents a refractive index of each lens with respect to ad-line, and ν represents an abbe number of each lens with respect to thed-line. The shape of an aspherical surface (in Table 4, such a surfaceis denoted with a mark * attached beside its reference number) isdefined by the aforementioned equation (80).

The following Table 5 shows aspherical coefficients of the zoom lens inthe present example.

TABLE 5 Surface k D E F G 8 −11.79950 −2.20951 × 10⁻³  1.33194 × 10⁻⁴−1.25908 × 10⁻⁵ 5.36379 × 10⁻⁷ 11 0.17661 −2.65165 × 10⁻⁴  6.26544 ×10⁻⁷  1.06422 × 10⁻⁷  1.35942 × 10⁻¹⁰ 12 0.10560  6.23500 × 10⁻⁴ 4.29405 × 10⁻⁶  6.88052 × 10⁻⁸  2.80861 × 10⁻¹⁰ 15 −30.31690  5.21270 ×10⁻⁴ −9.10874 × 10⁻⁶ −8.92635 × 10⁻⁷ 3.69895 × 10⁻⁸ 17 0.12809 −1.23533× 10⁻⁴  2.35203 × 10⁻⁵ −2.46202 × 10⁻⁶ 9.65532 × 10⁻⁸

The following Table 6 shows an air distance (mm) that is varied byzooming in the case where an object is positioned at infinity.

TABLE 6 Wide position Normal position Tele position Focal length 3.01025.627 68.915 F No. 1.688 2.490 3.355 Angle of 65.136 8.060 2.960 view(2ω) th5 0.700 16.925 20.316 th10 20.740 4.515 1.124 th12 8.120 2.6298.120 th17 2.000 7.491 2.000

The normal position in Table 6 is where the third lens group 23 isplaced most closely to the fourth lens group 24 In Table 6, Focal length(mm), F No., and ω(°) represent a focal length, an F number, and anincident angle of view at a wide position, a normal position, and a teleposition of the zoom lens of the present example.

FIGS. 6A to 6E, 7A to 7E, and 8A to 8E show performances regardingvarious aberrations at the wide position, the normal position, and thetele position of the zoom lens shown in the present example,respectively. FIGS. 6A, 7A and 8A show a spherical aberration (mm);FIGS. 6B, 7B and 8B show astigmatism (mm); FIGS. 6C, 7C and 8C show adistortion aberration (%); FIGS. 6D, 7D and 8D show a longitudinalchromatic aberration (mm); and FIGS. 6E, 7E and 8E show a chromaticaberration of magnification (mm). In FIGS. 6B, 7B and 8B showingastigmatism, a solid line represents a sagittal field curvature, and abroken line represents a meridional field curvature. In FIGS. 6D, 7D and8D showing the longitudinal chromatic aberration and FIGS. 6E, 7E and 8Eshowing the chromatic aberration of magnification, a solid linerepresents values with respect to the d-line, a short broken linerepresents values with respect to an F-line, and a long broken linerepresents values with respect to a C-line. As is apparent from thedrawings regarding these aberrations, the zoom lens of the presentexample has an excellent aberration performance.

A movement amount of the image plane position according to a change in arefractive index of a plastic lens material caused by a temperaturechange is 1.0 μm/C.° when the object is positioned at infinity and thezooming position is at the wide position.

[Embodiment 3]

FIG. 9 is a view showing the arrangement of a zoom lens according toEmbodiment 3 of the present invention.

As shown in FIG. 9, the zoom lens has a structure in which a first lensgroup 31, a second lens group 32, a third lens group 33, a fourth lensgroup 34, and a glass plate 35 are arranged from an object side (leftside in FIG. 9) to an image plane 36 side (right side in FIG. 9) in thisorder. Here, the glass plate 35 is equivalent optically to a crystalfilter or a face plate of an imaging device, etc.

The first lens group 31 has positive refracting power, and is fixed withrespect to the image plane 36 even when varying power and focusing. Thesecond lens group 32 has negative refracting power and varies power bymoving along an optical axis. The third lens group 33 has positiverefracting power, and is fixed with respect to the image plane 36 evenwhen varying power and focusing. The fourth lens group 34 has positiverefracting power, and moves along the optical axis so that the imageplane 36 varied by the movement of the second lens group 32 and themovement of the object to be imaged is kept at a predetermined positionfrom a reference plane, thereby moving an image and adjusting the focusthereof at the same time in accordance with variable power.

The first lens group 31 is composed of a negative lens 9 a, a positivelens 9 b, and a positive meniscus lens 9 c arranged from the object sidein this order, in which the positive meniscus lens 9 c has a convexsurface on the object side. The second lens group 32 is composed of anegative lens 10 a, and a cemented lens of a double-concave lens 10 band a positive lens 10 c, which are arranged from the object side inthis order, in which at least one of the surfaces of the foregoinglenses is aspherical. The third lens group 33 is composed of a positivelens 11 a and a negative plastic lens 11 b arranged from the object sidein this order, in which at least one of the surfaces of these lenses isaspherical. The fourth lens group 34 is composed of a negative plasticlens 12 a and a positive plastic lens 12 b that are arranged from theobject side in this order, in which at least one of the surfaces ofthese lenses is aspherical.

In the zoom lens according to the present embodiment, the followingexpression (90) desirably is satisfied:

5<|(fp 1 +fp 2 +fp 3)/fw|<12  (90)

where fp1 represents a focal length of the negative plastic lens 11 b ofthe third lens group 33, fp2 represents a focal length of the negativeplastic lens 12 a of the fourth lens group 34, fp3 represents a focallength of the positive plastic lens 12 b of the fourth lens group 34,and fw represents a combined focal length of the entire system at a wideposition.

With a configuration that satisfies the expression (90), changes inrefractive indices of the plastic lens materials caused by temperaturechanges can be canceled, whereby a deviation of the image plane positioncan be decreased. Generally, as properties of a plastic material, arefractive index thereof decreases as the temperature rises andincreases as the temperature falls, and the plastic material swells asthe temperature rises and shrinks as the temperature falls. In otherwords, if |(fp1+fp2+fp3)/fw| is not more than the lower limit of theexpression (90), a negative-lens tendency increases in the combinedfocal length of the focal length fp1 of the negative plastic lens 11 bof the third lens group 33, and the focal length fp2 of the negativeplastic lens 12 a and the focal length fp3 of the positive plastic lens12 b of the fourth lens group 34, and with a temperature rise, the imageplane position is deviated farthest on the object side at the wideposition. On the contrary, with a temperature fall, the image planeposition is deviated significantly toward the image plane side at thewide position. This causes a phenomenon in which the fourth lens group34 moving along the optical axis within a certain moving range so as tokeep the image plane at a predetermined position from a referencesurface is incapable of doing so as long as it moves within theforegoing moving range, thereby resulting in defocusing. On the otherhand, if |(fp1+fp2+fp3)/fw| is not less than the upper limit of theexpression (90), a positive-lens tendency increases in the combinedfocus length of the focus length fp1 of the negative plastic lens 11 bof the third lens group 33 and the focus length fp2 of the negativeplastic lens 12 a and the focus length fp3 of the positive plastic lens12 b of the fourth lens group 34, and with a temperature rise, the imageplane position is deviated farthest on the image plane side at thenormal position. Therefore, this also results in defocusing.

Furthermore, in the zoom lens of the present embodiment, the followingexpression (91) desirably is satisfied.

7<|(fp 1 +fp 2 +fp 3)/fw|<10.5  (91)

By configuring the zoom lens so that the expression (91) above issatisfied, changes in refractive indices of the plastic lens materialscaused by temperature changes are canceled, resulting in substantiallyno deviation of the image plane position occurring.

Furthermore, in the zoom lens according to the present embodiment, thefollowing expressions (92) to (95) desirably are satisfied:

9<f 1 /fw<11  (92)

1<|f 2 /fw|<2  (93)

4.5<f 3/fw<6  (94)

4.5<f 4 /fw<6.5  (95)

where f1 represents a combined focal length of the first lens group 31,f2 represents a combined focal length of the second lens group 32, f3represents a combined focal length of the third lens group 33, and f4represents a combined focal length of the fourth lens group 34.

In the case where the expressions (92) to (95) are satisfied, the zoomlens is configured to be compact, with aberration performancesexcellently adjusted.

If f1/fw is not more than the lower limit of the expression (92), thefirst lens group 31 has an excessive refracting power, which makes itdifficult to correct a spherical aberration at the side of the longfocal length and an off-axis coma-aberration. Besides, if f1/fw is notless than the upper limit of the expression (92), the full length of thelens increases, which makes it difficult to make the zoom lens compact.

If |f2/fw| is not more than the lower limit of the expression (93), aPetzval sum of the entire system increases, so that a field curvaturecannot be corrected. If |f2/fw| is not less than the upper limit of theexpression (93), the Petzval sum decreases, but the length of the entiresystem increases, which makes it difficult to make the zoom lenscompact.

If f3/fw is not more than the lower limit of the expression (94), therefracting power of the third lens group 33 increases, which makes itimpossible to secure a back-focus that allows a crystal filter or thelike to be inserted therein, and makes it difficult to correct thespherical aberration. Furthermore, if f3/fw is not less than the upperlimit of the expression (94), a Petzval sum increases, thereby making itdifficult to correct a field curvature.

If f4/fw is not more than the lower limit of the expression (95), thesize of the entire lens system increases, which makes it difficult tomake the zoom lens compact. Furthermore, if f4/fw is not less than theupper limit of the expression (95), it is difficult to correct off-axisaberrations both in near photographing and in long-distancephotographing at the same time

Furthermore, in the zoom lens according to the present embodiment, thefollowing expression (96) desirably is satisfied:

d 12 ×fw <1.2  (96)

where d12 represents a distance between the positive lens 11 a and thenegative plastic lens 11 b of the third lens group 33.

In the case where the expression (96) is satisfied, a chromaticaberration can be corrected excellently in a zooming range from the wideposition to the tele position. If d12×fw is not less than the upperlimit of the expression (96), the chromatic aberration significantlyvaries from the wide position to the tele position, therebysignificantly deteriorating the performance.

In the zoom lens according to the present embodiment, the followingexpression (97) desirably is satisfied:

(sag(r 1)+sag(r 2)+d 8)/d 8<4.5  (97)

where

sag (r1) represents a sag amount between the center of an incidentsurface of the double-concave lens 10 b of the second lens group 32 anda position where the incident surface of the double-concave lens 10 b isbrought into contact with an outgoing surface of the negative lens 10 adisposed on the object side in the second lens group 32,

sag (r2) represents a sag amount between the center and an outer-mostperipheral portion of the outgoing surface of the double-concave lens 10b, and,

d8 denotes a thickness of the double-concave lens 10 b.

With satisfaction of the expression (97), the double-concave lens 10 bcan be formed readily, whereby the yield thereof can be improved. If(sag(r1)+sag(r2)+d8)/d8 is not less than the upper limit of theexpression (97), the ratio of a thickness of the central portion of thelens to an edge thickness of the peripheral portion of the lensincreases, making it difficult to mold a lens. As a result, the yield islowered and a low cost of lenses cannot be realized.

Furthermore, desirably the zoom lens according to the present embodimentis configured so that a lens surface closest to the image plane of thefirst lens group 31 has a radius of curvature equal to a radius ofcurvature of a lens surface closest to the object of the second lensgroup 32. This prevents the distance between the surface closest to theimage plane of the first lens group 31 and the surface closest to theobject of the second lens group 32 from decreasing with increasingproximity to a lens periphery. This facilitates the production of a lensbarrel.

Furthermore, in the zoom lens according to the present embodiment, thefollowing expression (98) desirably is satisfied:

0.6<BF/fw<1.1  (98)

where BF represents an air distance between the image-plane-side surfaceof the lens closest to the image plane and the image plane.

By satisfying the foregoing expression (98), it is possible to ensure aback-focus necessary for allowing an infrared cut-off filter or alow-pass filter such as a crystal filter to be inserted. Besides, theback-focus is prevented from increasing unnecessarily, which makes itpossible to provide a compact zoom lens. If BF/fw is not more than thelower limit of the expression (98), a distance sufficient for allowingan infrared cut-off filter or a low-pass filter such as a crystal filterto be inserted cannot be ensured. On the other hand, if BF/fw is notless than the upper limit of the expression (98), the back-focusexcessively increases, thereby making it impossible to provide a compactzoom lens.

EXAMPLE 3

The following Table 7 shows a specific example of the zoom lensaccording to the present embodiment.

TABLE 7 Group Surface rd th nd ν 1  1 39.25 0.80 1.80518 25.4  2 20.475.10 1.58913 61.2  3 −171.50 0.20  4 18.24 2.75 1.60311 60.7  5 45.42variable 2  6 45.42 0.60 1.80500 39.6  7 4.30 2.70  8* −8.58 0.901.60602 57.8  9 5.51 2.30 1.80518 25.5 10 73.39 variable 3 11* 8.63 3.801.60602 57.8 12* −9.39 0.20 13 −13.38 0.70 1.58387 30.1 14 16.47variable 4 15* 10.28 1.00 1.58387 30.1 16 6.00 0.30 17 5.70 2.80 1.4917857.2 18* −22.98 variable 5 19 ∞ 2.80 1.51633 64.1 20 ∞

In Table 7, rd represents a radius of curvature (mm) of a lens, threpresents a thickness (mm) of a lens or an air distance (mm) betweenlenses, nd represents a refractive index of each lens with respect to ad-line, and ν represents an abbe number of each lens with respect to thed-line. The shape of an aspherical surface (in Table 7, such a surfaceis denoted with a mark * attached beside its reference number) isdefined by the aforementioned equation (80).

The following Table 8 shows aspherical coefficients of the zoom lens inthe present example.

TABLE 8 Surface k D E F G 8 −11.79950 −2.20951 × 10⁻³  1.33194 × 10⁻⁴−1.25908 × 10⁻⁵  5.36379 × 10⁻⁷ 11 0.69201 −2.54836 × 10⁻⁴ −3.96421 ×10⁻⁵  3.21063 × 10⁻⁷ −6.30435 × 10⁻⁹ 12 0.49478  5.43522 × 10⁻⁴  3.05097× 10⁻⁶  2.39230 × 10⁻⁷ −4.48837 × 10⁻⁹ 17 −0.44842  9.83921 × 10⁻⁵ 6.00419 × 10⁻⁶  1.99002 × 10⁻⁸ −9.74119 × 10⁻⁸ 18 −108.49600 −6.70268 ×10⁻⁴  8.89076 × 10⁻⁵ −1.15393 × 10⁻⁶ −4.33822 × 10⁻⁸

The following Table 9 shows an air distance (mm) that is varied byzooming in the case where an object is positioned at infinity.

TABLE 9 Wide position Normal position Tele position Focal length 3.01026.710 69.512 F No. 1.688 2.485 3.385 Angle of 65.136 7.730 2.948 view(2ω) th5 0.700 16.950 20.341 th10 20.740 4.412 1.099 th12 8.120 2.5388.120 th17 2.000 7.582 2.000

The normal position in Table 9 is where the third lens group 33 isplaced most closely to the fourth lens group 34 In Table 9, Focal length(mm), F No., and ω(°) represent a focal length, an F number, and anincident angle of view at a wide position, a normal position, and a teleposition of the zoom lens of the present example.

FIGS. 10A to 10E, 11A to 11E, and 12A to 12E show performances regardingvarious aberrations at the wide position, the normal position, and thetele position of the zoom lens shown in the present example,respectively, FIGS. 10A, 11A and 12A show a spherical aberration (mm);FIGS. 10B, 11B and 12B show astigmatism (mm); FIGS. 10C, 11C and 12Cshow a distortion aberration (%); FIGS. 10D, 11D and 12D show alongitudinal chromatic aberration (mm); and FIGS. 10E, 11E and 12E showa chromatic aberration of magnification (mm). In FIGS. 10B, 11B and 12Bshowing astigmatism, a solid line represents a sagittal field curvature,and a broken line represents a meridional field curvature. In FIGS. 10D,11D and 12D showing the longitudinal chromatic aberration and FIGS. 10E,11E and 12E showing the chromatic aberration of magnification, a solidline represents values with respect to the d-line, a short broken linerepresents values with respect to an F-line, and a long broken linerepresents values with respect to a C-line. As is apparent from thedrawings regarding these aberrations, the zoom lens of the presentexample has an excellent aberration performance.

A movement amount of the image plane position according to a change in arefractive index of a plastic lens material caused by a temperaturechange is 1.2 μm/C.° when the object is positioned at infinity and thezooming position is at the wide position.

[Embodiment 4]

FIG. 13 is a view showing the arrangement of a zoom lens according toEmbodiment 4 of the present invention.

As shown in FIG. 13, the zoom lens has a structure in which a first lensgroup 41, a second lens group 42, a third lens group 43, a fourth lensgroup 44, and a glass plate 45 are arranged from an object side (leftside in FIG. 13) to an image plane 46 side (right side in FIG. 13) inthis order. Here, the glass plate 45 is equivalent optically to acrystal filter or a face plate of an imaging device, etc.

The first lens group 41 has positive refracting power, and is fixed withrespect to the image plane 46 even when varying power and focusing. Thesecond lens group 42 has negative refracting power and varies power bymoving along an optical axis. The third lens group 43 has positiverefracting power, and is fixed with respect to the image plane 46 evenwhen varying power and focusing. The fourth lens group 44 has positiverefracting power, and moves along the optical axis so that the imageplane 46 varied by the movement of the second lens group 42 and themovement of the object to be imaged is kept at a predetermined positionfrom a reference plane, thereby moving an image and adjusting the focusthereof at the same time in accordance with variable power.

The first lens group 41 is composed of a negative lens 13 a, a positivelens 13 b, and a positive meniscus lens 13 c arranged from the objectside in this order, in which the positive meniscus lens 13 c has aconvex surface on the object side. The second lens group 42 is composedof a negative lens 14 a, and a cemented lens of a double-concave lens 14b and a positive lens 14 c, which are arranged from the object side inthis order, in which at least one of the surfaces of the foregoinglenses is aspherical. The third lens group 43 is a cemented lenscomposed of a positive lens 15 a and a negative plastic lens 15 barranged from the object side in this order, in which at least one ofthe surfaces of these lenses is aspherical. The fourth lens group 44 isa cemented lens composed of a negative plastic lens 16 a and a positiveplastic lens 16 b that are arranged from the object side in this order,in which at least one of the surfaces of these lenses is aspherical.

In the zoom lens according to the present embodiment, the followingexpression (99) is satisfied:

5<|(fp 1 +fp 2 +fp 3)/fw|<12   (99)

where fp1 represents a focal length of the negative plastic lens 15 b ofthe third lens group 43, fp2 represents a focal length of the negativeplastic lens 16 a of the fourth lens group 44, fp3 represents a focallength of the positive plastic lens 16 b of the fourth lens group 44,and fw represents a combined focal length of the entire system at a wideposition.

With such a configuration that satisfies the expression (99), changes inrefractive indices of the plastic lens materials caused by temperaturechanges can be canceled, whereby a deviation of the image plane positioncan be decreased. Generally, as properties of a plastic material, arefractive index thereof decreases as the temperature rises andincreases as the temperature falls, and the plastic material swells asthe temperature rises and shrinks as the temperature falls. In otherwords, if |(fp1+fp2+fp3)/fw| is not more than the lower limit of theexpression (99), a negative-lens tendency increases in the combinedfocal length of the focal length fp1 of the negative plastic lens 15 bof the third lens group 43, and the focal length fp2 of the negativeplastic lens 16 a and the focal length fp3 of the positive plastic lens16 b of the fourth lens group: 44, and with a temperature rise, theimage plane position is deviated farthest on the object side at the wideposition. On the contrary, with a temperature fall, the image planeposition is deviated significantly toward the image plane side at thewide position. This causes a phenomenon in which the fourth lens group44 moving along the optical axis within a certain moving range so as tokeep the image plane at a predetermined position from a referencesurface is incapable of doing so as long as it moves within theforegoing moving range, thereby resulting in defocusing. On the otherhand, if |(fp1+fp2+fp3)/fw| is not less than the upper limit of theexpression (99), a positive-lens tendency increases in the combinedfocus length of the focus length fp1 of the negative plastic lens 15 bof the third lens group 43 and the focus length fp2 of the negativeplastic lens 16 a and the focus length fp3 of the positive plastic lens16 b of the fourth lens group 44, and with a temperature rise, the imageplane position is deviated farthest on the image plane side at thenormal position. Therefore, this also results in defocusing.

Furthermore, in the zoom lens of the present embodiment, the followingexpression (100) desirably is satisfied.

7<|(fp 1 +fp 2 +fp 3)/fw|<10.5  (100)

With such a configuration of the zoom lens that satisfies the expression(100) above, changes in refractive indices of the plastic lens materialscaused by temperature changes are canceled, resulting in substantiallyno deviation of the image plane position occurring.

Furthermore, in the zoom lens according to the present embodiment, thefollowing expressions (101) to (104) desirably are satisfied:

9<f 1 /fw<11  (101)

1<|f 2 /fw|<2  (102)

4.5<f 3/fw<6  (103)

4.5<f 4/fw<6.5  (104)

where f1 represents a combined focal length of the first lens group 41,f2 represents a combined focal length of the second lens group 42, f3represents a combined focal length of the third lens group 43, and f4represents a combined focal length of the fourth lens group 44.

In the case where the expressions (101) to (104) are satisfied, the zoomlens is configured to be compact, with aberration performancesexcellently adjusted.

If f1/fw is not more than the lower limit of the expression (101), thefirst lens group 41 has an excessive refracting power, which makes itdifficult to correct a spherical aberration at the side of the longfocal length and an off-axis coma-aberration. Besides, if f1/fw is notless than the upper limit of the expression (101), the full length ofthe lens increases, which makes it difficult to make the zoom lenscompact.

If |f2/fw| is not more than the lower limit of the expression (102), aPetzval sum of the entire system increases, so that a field curvaturecannot be corrected. If |f2/fw| is not less than the upper limit of theexpression (102), the Petzval sum decreases, but the length of theentire system increases, which makes it difficult to make the zoom lenscompact.

If f3/fw is not more than the lower limit of the expression (103), therefracting power of the third lens group 43 increases, which makes itimpossible to secure a back-focus that allows a crystal filter or thelike to be inserted therein, and makes it difficult to correct thespherical aberration. Furthermore, if f3/fw is not less than the upperlimit of the expression (103), a Petzval sum increases, thereby makingit difficult to correct a field curvature.

If f4/fw is not more than the lower limit of the expression (104), thesize of the entire lens system increases, which makes it difficult tomake the zoom lens compact. Furthermore, if f4/fw is not less than theupper limit of the expression (104), it is difficult to correct off-axisaberrations both in near photographing and in long-distancephotographing at the same time.

In the zoom lens according to the present embodiment, the followingexpression (105) desirably is satisfied:

(sag(r 1)+sag(r 2)+d 8)/d 8<4.5  (105)

where

sag (r1) represents a sag amount between the center of an incidentsurface of the double-concave lens 14 b of the second lens group 42 anda position where the incident surface of the double-concave lens 14 b isbrought into contact with an outgoing surface of the negative lens 14 adisposed on the object side in the second lens group 42,

sag (r2) represents a sag amount between the center and an outer-mostperipheral portion of the outgoing surface of the double-concave lens 14b, and

d8 denotes a thickness of the double-concave lens 14 b.

With satisfaction of the expression (105), the double-concave lens 14 bcan be formed readily, whereby the yield thereof can be improved. If(sag(r1)+sag(r2)+d8)/d8 is not less than the upper limit of theexpression (105), the ratio of a thickness of the central portion of thelens to an edge thickness of the peripheral portion of the lensincreases, making it difficult to mold a lens. As a result, the yield islowered and a low cost of lenses cannot be realized.

Furthermore, desirably the zoom lens according to the present embodimentis configured so that a lens surface closest to the image plane of thefirst lens group 41 has a radius of curvature equal to a radius ofcurvature of a lens surface closest to the object of the second lensgroup 42. This prevents the distance between the surface closest to theimage plane of the first lens group 41 and the surface closest to theobject of the second lens group 42 from decreasing with increasingproximity to a lens periphery. This facilitates the production of a lensbarrel.

Furthermore, in the zoom lens according to the present embodiment, thefollowing expression (106) desirably is satisfied:

0.6<BF/fw<1.1  (106)

where BF represents an air distance between the image-plane-side surfaceof the lens closest to the image plane of lens and the image plane.

By satisfying the foregoing expression (106), it is possible to ensure aback-focus necessary for allowing an infrared cut-off filter or alow-pass filter such as a crystal filter to be inserted. Besides, theback-focus is prevented from increasing unnecessarily, which makes itpossible to provide. a compact zoom lens. If BF/fw is not more than thelower limit of the expression (106), a distance sufficient for allowingan infrared cut-off filter or a low-pass filter such as a crystal filterto be inserted cannot be ensured. On the other hand, if BF/fw is notless than the upper limit of the expression (106), the back-focusexcessively increases, thereby making it impossible to provide a compactzoom lens.

EXAMPLE 4

The following Table 10 shows a specific example of the zoom lensaccording to the present embodiment.

TABLE 10 Group Surface rd th nd ν 1  1 37.65 0.80 1.80518 25.4  2 20.215.05 1.58913 61.2  3 −234.89 0.15  4 18.83 2.75 1.60311 60.7  5 50.63variable 2  6 50.63 0.60 1.80500 39.6  7 4.37 2.71  8* −8.55 0.801.60602 57.8  9 5.48 2.20 1.80518 25.5 10 72.19 variable 3 11* 8.11 3.701.60602 57.8 12 −13.22 0.60 1.58387 30.1 13* 42.40 variable 4 14* 6.900.60 1.58387 30.1 15 3.89 2.70 1.49178 57.2 16* 100.22 variable 17 ∞2.80 1.51633 64.1 5 18 ∞ —

In Table 10, rd represents a radius of curvature (mm) of a lens, threpresents a thickness (mm) of a lens or an air distance (mm) betweenlenses, nd represents a refractive index of each lens with respect to ad-line, and ν represents an abbe number of each lens with respect to thed-line. The shape of an aspherical surface (in Table 10, such a surfaceis denoted with a mark * attached beside its reference number) isdefined by the aforementioned equation (80).

The following Table 11 shows aspherical coefficients of the zoom lens inthe present example.

TABLE 11 Surface k D E F G 8 −11.84580 −2.22011 × 10⁻³ 1.32305 × 10⁻⁴−1.26272 × 10⁻⁵  5.38080 × 10⁻⁷ 11 0.72114 −1.21990 × 10⁻⁴ 3.28842 ×10⁻⁷  3.36737 × 10⁻⁷ −1.10588 × 10⁻⁸ 12 30.05691  4.72068 × 10⁻⁴ 1.40761× 10⁻⁵  8.39921 × 10⁻⁷ −7.60437 × 10⁻⁹ 14 −0.04235  8.01700 × 10⁻⁵3.49848 × 10⁻⁵  4.26612 × 10⁻⁷ −4.73729 × 10⁻⁸ 16 263.25400  4.07337 ×10⁻⁴ 8.48037 × 10⁻⁵ −6.68023 × 10⁻⁷ −1.49323 × 10⁻⁷

The following Table 12 shows an air distance (mm) that is varied byzooming in the case where an object is positioned at infinity.

TABLE 12 Wide position Normal position Tele position Focal length 3.01028.046 69.068 F No. 1.688 2.450 3.373 Angle of 65.136 7.300 2.960 view(2ω) th5 0.700 16.949 20.341 th10 20.740 4.491 1.099 th12 8.120 2.0558.120 th17 2.000 8.065 2.000

The normal position in Table 12 is where the third lens group 43 isplaced most closely to the fourth lens group 44 In Table 12, Focallength (mm), F No., and ω(°) represent a focal length, an F number, andan incident angle of view at a wide position, a normal position, and atele position of the zoom lens of the present example.

FIGS. 14A to 14E, 15A to 15E, and 16A to 16E show performances regardingvarious aberrations at the wide position, the normal position, and thetele position of the zoom lens shown in the present example,respectively. FIGS. 14A, 15A and 16A show a spherical aberration (mm);FIGS. 14B, 15B and 16B show astigmatism (mm); FIGS. 14C, 15C and 16Cshow a distortion aberration (%); FIGS. 14D, 15D and 16D show alongitudinal chromatic aberration (mm); and FIGS. 14E, 15E and 16E showa chromatic aberration of magnification (mm). In FIGS. 14B, 15B and 16Bshowing astigmatism, a solid line represents a sagittal field curvature,and a broken line represents a meridional field curvature. In FIGS. 14D,15D and 16D showing the longitudinal chromatic aberration and FIGS. 14E,15E and 16E showing the chromatic aberration of magnification, a solidline represents values with respect to the d-line, a short broken linerepresents values with respect to an F-line, and a long broken linerepresents values with respect to a C-line. As is apparent from thedrawings regarding these aberrations, the zoom lens of the presentexample has an excellent aberration performance.

A movement amount of the image plane position according to a change in arefractive index of a plastic lens material caused by a temperaturechange is 0.9 μm/C.° when the object is positioned at infinity and thezooming position is at the wide position.

[Embodiment 5]

FIG. 17 is a view showing an arrangement of the configuration of a videocamera according to the fifth embodiment of the present invention.

As shown in FIG. 17, the video camera according to this embodimentincludes a zoom lens 100, a low-pass filter 101, an imaging device 102,a signal processing circuit 103, a viewer finder 104 and a recordingsystem 105. Herein, as the zoom lens 100, the zoom lens according toEmbodiment 1 is used.

In the case where a video camera is configured using a zoom lens of thepresent. invention, it is possible to provide a video camera thatachieves high performance and low cost, as well as a high magnificationat a zoom ratio of 23 times. It should be noted that even in the casewhere any one of the zoom lenses of Embodiments 2 to 4 are used, it ispossible to provide a video camera that achieves high performance andlow cost, as well as a high magnification at a zoom ratio of 23 times.

INDUSTRIAL APPLICABILITY

As mentioned above, according to the present invention, it is possibleto provide a zoom lens that achieves high brightness at an F number of1.6, a high magnification at a zoom ratio of 23 times, as well as highperformance and low cost. Therefore, the zoom lens is applicable in avideo camera that is requested to achieve a high zoom ratio, highfunctionality, and low cost.

What is claimed is:
 1. A zoom lens, comprising: a first lens grouphaving positive refracting power and being fixed with respect to theimage plane; a second lens group having negative refracting power andvarying power by moving along an optical axis; a third lens group havingpositive refracting power and being fixed with respect to the imageplane; and a fourth lens group having positive refracting power andmoving along the optical axis so that the image plane varied by amovement of the second lens group and a movement of an object is kept ata predetermined position from a reference plane, the first, second,third, and fourth lens groups being arranged in this order from anobject side to an image plane side, wherein the first lens groupcomprises a negative lens, a positive lens, and a positive meniscus lensarranged from the object side in this order, the positive meniscus lenshaving a convex surface on the object side, the second lens groupcomprises a negative lens, a double-concave lens, and a positive lensarranged from the object side in this order, and includes at least oneaspherical surface, the double-concave lens and the positive lens beingcemented with each other, the third lens group comprises a positive lensand a negative plastic lens arranged from the object side in this order,and includes at least one aspherical surface, and the fourth lens groupcomprises a negative plastic lens and a positive plastic lens that arearranged from the object side in this order and cemented with eachother, and includes at least one aspherical surface, wherein anexpression (1) below is satisfied: 5<|(fp 1 +fp 2 +fp 3)/fw|<12  (1) where fp1 represents a focal length of the negative plastic lens of thethird lens group, fp2 represents a focal length of the negative plasticlens of the fourth lens group, fp3 represents a focal length of thepositive plastic lens of the fourth lens group, and fw represents acombined focal length of the entire system at a wide position.
 2. Thezoom lens according to claim 1, wherein an expression (2) below issatisfied: 7<|(fp 1+fp 2 +fp 3)/fw|<10.5  (2).
 3. The zoom lensaccording to claim 2, wherein expressions (3) to (6) below aresatisfied: 9<f 1 /fw<11  (3) 1<|f 2 /fw|<2  (4) 4.5<f 3 /fw<6  (5) 4.5<f4 /fw<6.5  (6) where f1 represents a combined focal length of the firstlens group, f2 represents a combined focal length of the second lensgroup, f3 represents a combined focal length of the third lens group,and f4 represents a combined focal length of the fourth lens group. 4.The zoom lens according to claim 3, wherein an expression (7) below issatisfied: d 12 ×fw<1.2  (7) where d12 represents a distance between thepositive lens and the negative plastic lens of the third lens group. 5.The zoom lens according to claim 1, wherein an expression (8) below issatisfied: (sag(r 1)+sag(r 2)+d 8)/d 8<4.5  (8) where sag(r1) representsa sag amount between a center of an incident surface of thedouble-concave lens of the second lens group and a position where theincident surface of the double-concave lens is brought into contact withan outgoing surface of the negative lens disposed on the object side inthe second lens group, sag (r2) represents a sag amount between a centerand an outer-most peripheral portion of the outgoing surface of thedouble-concave lens, and d8 denotes a thickness of the double-concavelens.
 6. The zoom lens according to claim 1, wherein a radius ofcurvature of a lens surface closest to the image plane of the first lensgroup and a radius of curvature of a lens surface closest to the objectof the second lens group are equal to each other.
 7. The zoom lensaccording to claim 1, wherein an expression (9) below is satisfied:0.6<BF/fw<1.1  (9) where BF represents an air distance between animage-plane-side surface of the lens closest to the image plane and theimage plane.
 8. A zoom lens, comprising: a first lens group havingpositive refracting power and being fixed with respect to the imageplane; a second lens group having negative refracting power and varyingpower by moving along an optical axis; a third lens group havingpositive refracting power and being fixed with respect to the imageplane; and a fourth lens group having positive refracting power andmoving along the optical axis so that the image plane varied by amovement of the second lens group and a movement of an object is kept ata predetermined position from a reference plane, the first, second,third, and fourth lens groups being arranged in this order from anobject side to an image plane side, wherein the first lens groupcomprises a negative lens, a positive lens, and a positive meniscus lensarranged from the object side in this order, the positive meniscus lenshaving a convex surface on the object side, the second lens groupcomprises a negative lens, a double-concave lens, and a positive lensarranged from the object side in this order, and includes at least oneaspherical surface, the double-concave lens and the positive lens beingcemented with each other, the third lens group comprises a positive lensand a negative plastic lens arranged from the object side in this order,and includes at least one aspherical surface, and the fourth lens groupcomprises a positive plastic lens and a negative plastic lens that arearranged from the object side in this order and cemented with eachother, and includes at least one aspherical surface, wherein anexpression (10) below is satisfied: 5<|(fp 1 +fp 2 +fp 3)/fw1<12  (10) where fp1 represents a focal length of the negative plastic lens of thethird lens group, fp2 represents a focal length of the positive plasticlens of the fourth lens group, fp3 represents a focal length of thenegative plastic lens of the fourth lens group, and fw represents acombined focal length of the entire system at a wide position.
 9. Thezoom lens according to claim 8, wherein an expression (11) below issatisfied: 7<|(fp 1 +fp 2 +fp 3)/fw|<10.5  (11).
 10. The zoom lensaccording to claim 9, wherein expressions (12) to (15) below aresatisfied: 9<f 1 /fw<11  (12) 1<|f 2 /fw|<2  (13) 4.5<f 3 /fw<6  (14)4.5<f 4 /fw<6.5  (15) where f1 represents a combined focal length of thefirst lens group, f2 represents a combined focal length of the secondlens group, f3 represents a combined focal length of the third lensgroup, and f4 represents a combined focal length of the fourth lensgroup.
 11. The zoom lens according to claim 10, wherein an expression(16) below is satisfied: d 12 ×fw<1.2  (16) where d12 represents adistance between the positive lens and the negative plastic lens of thethird lens group.
 12. The zoom lens according to claim 8, wherein anexpression (17) below is satisfied: (sag(r 1)+sag(r 2)+d 8)/d8<4.5  (17) where sag (r1) represents a sag amount between a center ofan incident surface of the double-concave lens of the second lens groupand a position where the incident surface of the double-concave lens isbrought into contact with an outgoing surface of the negative lensdisposed on the object side in the second lens group, sag (r2)represents a sag amount between a center and an outer-most peripheralportion of the outgoing surface of the double-concave lens, and d8denotes a thickness of the double-concave lens.
 13. The zoom lensaccording to claim 8, wherein a radius of curvature of a lens surfaceclosest to the image plane of the first lens group and a radius ofcurvature of a lens surface closest to the object of the second lensgroup are equal to each other.
 14. The zoom lens according to claim 8,wherein an expression (18) below is satisfied: 0.6<BF/fw<1.1  (18) whereBF represents an air distance between an image-plane-side surface of thelens closest to the image plane and the image plane.
 15. A zoom lens,comprising: a first lens group having positive refracting power andbeing fixed with respect to the image plane; a second lens group havingnegative refracting power and varying power by moving along an opticalaxis; a third lens group having positive refracting power and beingfixed with respect to the image plane; and a fourth lens group havingpositive refracting power and moving along the optical axis so that theimage plane varied by a movement of the second lens group and a movementof an object is kept at a predetermined position from a reference plane,the first, second, third, and fourth lens groups being arranged in thisorder from an object side to an image plane side, wherein the first lensgroup comprises a negative lens, a positive lens, and a positivemeniscus lens arranged from the object side in this order, the positivemeniscus lens having a convex surface on the object side, the secondlens group comprises a negative lens, a double-concave lens, and apositive lens arranged from the object side in this order, and includesat least one aspherical surface, the double-concave lens and thepositive lens being cemented with each other, the third lens groupcomprises a positive lens and a negative plastic lens arranged from theobject side in this order, and includes at least one aspherical surface,and the fourth lens group comprises a negative plastic lens and apositive plastic lens that are arranged from the object side in thisorder, and includes at least one aspherical surface, wherein anexpression (19) below is satisfied: 5<|(fp 1 +fp 2 +fp 3)/fw|<12  (19) where fp1 represents a focal length of the negative plastic lens of thethird lens group, fp2 represents a focal length of the negative plasticlens of the fourth lens group, fp3 represents a focal length of thepositive plastic lens of the fourth lens group, and fw represents acombined focal length of the entire system at a wide position.
 16. Thezoom lens according to claim 15, wherein an expression (20) below issatisfied: 7<|(fp 1 +fp 2 +fp 3)/fw|<10.5  (20).
 17. The zoom lensaccording to claim 16, wherein expressions (21) to (24) below aresatisfied: 9<f 1 /fw<11  (21) 1<|f 2 /fw|<2  (22) 4.5<f 3 /fw<6  (23)4.5<f 4/fw<6.5  (24) where f1 represents a combined focal length of thefirst lens group, f2 represents a combined focal length of the secondlens group, f3 represents a combined focal length of the third lensgroup, and f4 represents a combined focal length of the fourth lensgroup.
 18. The zoom lens according to claim 17, wherein an expression(25) below is satisfied: d 12 ×fw<1.2  (25) where d12 represents adistance between the positive lens and the negative plastic lens of thethird lens group.
 19. The zoom lens according to claim 15, wherein anexpression (26) below is satisfied: (sag(r 1)+sag(r 2)+d 8)/d8<4.5  (26) where sag(rl) represents a sag amount between a center of anincident surface of the double-concave lens of the second lens group anda position where the incident surface of the double-concave lens isbrought into contact with an outgoing surface of the negative lensdisposed on the object side in the second lens group, sag(r2) representsa sag amount between a center and an outer-most peripheral portion ofthe outgoing surface of the double-concave lens, and d8 denotes athickness of the double-concave lens.
 20. The zoom lens according toclaim 15, wherein a radius of curvature of a lens surface closest to theimage plane of the first lens group and a radius of curvature of a lenssurface closest to the object of the second lens group are equal to eachother.
 21. The zoom lens according to claim 15, wherein an expression(27) below is satisfied: 0.6<BF/fw<1.1  (27) where BF represents an airdistance between an image-plane-side surface of the lens closest to theimage plane and the image plane.
 22. A zoom lens, comprising: a firstlens group having positive refracting power and being fixed with respectto the image plane; a second lens group having negative refracting powerand varying power by moving along an optical axis; a third lens grouphaving positive refracting power and being fixed with respect to theimage plane; and a fourth lens group having positive refracting powerand moving along the optical axis so that the image plane varied by amovement of the second lens group and a movement of an object is kept ata predetermined position from a reference plane, the first, second,third, and fourth lens groups being arranged in this order from anobject side to an image plane side, wherein the first lens groupcomprises a negative lens, a positive lens, and a positive meniscus lensarranged from the object side in this order, the positive meniscus lenshaving a convex surface on the object side, the second lens groupcomprises a negative lens, a double-concave lens, and a positive lensarranged from the object side in this order, and includes at least oneaspherical surface, the double-concave lens and the positive lens beingcemented with each other, the third lens group comprises a positive lensand a negative plastic lens arranged from the object side in this orderand cemented with each other, and includes at least one asphericalsurface, and the fourth lens group comprises a negative plastic lens anda positive plastic lens that are arranged from the object side in thisorder and cemented with each other, and includes at least one asphericalsurface, wherein an expression (1) below is satisfied: 5<|(fp 1+fp 2+fp3)/fw|<12  (1)  where fp1 represents a focal length of the negativeplastic lens of the third lens group, fp2 represents a focal length ofthe negative plastic lens of the fourth lens group, fp3 represents afocal length of the positive plastic lens of the fourth lens group, andfw represents a combined focal length of the entire system at a wideposition.
 23. The zoom lens according to claim 22, wherein an expression(29) below is satisfied: 7<|(fp 1 +fp 2 +fp 3)/fw|<10.5  (29).
 24. Thezoom lens according to claim 23, wherein expressions (30) to (33) beloware satisfied: 9<f 1 /fw<11  (30) 1<|f 2/fw|<2  (31) 4.5<f 3 /fw<6  (32)4.5<f 4/fw<6.5  (33) where f1 represents a combined focal length of thefirst lens group, f2 represents a combined focal length of the secondlens group, f3 represents a combined focal length of the third lensgroup, and f4 represents a combined focal length of the fourth lensgroup.
 25. The zoom lens according to claim 22, wherein an expression(34) below is satisfied: (sag(r 1)+sag(r 2)+d 8)/d 8<4,5  (34) wheresag(rl) represents a sag amount between a center of an incident surfaceof the double-concave lens of the second lens group and a position wherethe incident surface of the double-concave lens is brought into contactwith an outgoing surface of the negative lens disposed on the objectside in the second lens group, sag(r2) represents a sag amount between acenter and an outer-most peripheral portion of the outgoing surface ofthe double-concave lens, and d8 denotes a thickness of thedouble-concave lens.
 26. The zoom lens according to claim 22, wherein aradius of curvature of a lens surface closest to the image plane of thefirst lens group and a radius of curvature of a lens surface closest tothe object of the second lens group are equal to each other.
 27. Thezoom lens according to claim 22, wherein an expression (35) below issatisfied: 0.6<BF/fw<1.1  (35) where BF represents an air distancebetween an image-plane-side surface of the lens closest to the imageplane and the image plane.
 28. A video camera provided with the zoomlens of claim
 1. 29. A video camera provided with the zoom lens of claim8.
 30. A video camera provided with the zoom lens of claim
 15. 31. Avideo camera provided with the zoom lens of claim 22.